1. Field of the Invention
The present invention relates to a liquid crystal display device, and particularly to a liquid crystal display device and liquid crystal driving method which enable to drive a passive matrix liquid crystal display device at low power consumption and with high-display quality.
2. Description of the Related Art
A plural line selection driving system disclosed, for example, in Japanese Patent Laid-open Publication (Tokkai-Hei) No. 6-67628 is listed as a driving system which drives a liquid crystal display including a passive matrix liquid crystal panel. A prior art on the 2-line selection driving operation will be described below.
As shown in FIG. 2, a selection voltage is applied to scanning electrodes Yi (i=1 to n) corresponding to a row of a liquid crystal panel. The selection voltage is applied to each pairs of the scanning electrodes with a period of 2t which is sequentially shifted one pair of the electrodes to the next. The period t is expressed by the following formula: EQU t=(1/n).times.f (1)
where n is the number of lines per screen and f is the duration of a frame period being a time to scan all the lines per screen.
Some type of liquid crystal controllers which output image data to be displayed on a liquid crystal panel set, one frame period to be consisting of a display interval in which the selection voltage is applied to all the scanning electrodes and a retrace line interval (to be called "blank interval" hereafter) in which the selection voltage is not applied to any of the scanning electrodes.
In such a case, it is assumed in this specification that the frame period f corresponds to the display interval and the period t corresponds to a value of {(the display interval)/(the total number of the scanning electrodes)}.
The selection voltage has two levels, namely a positive level and a negative level with respect to a non-selection voltage level. The polarities of a selection voltage have an orthogonality between two selected scanning electrodes. In the 2-line selection driving system, the period 2t is divided into two t periods. A selection voltage having different polarities in the earlier half and later half of the selection period is applied to one of the selected scanning electrodes. A selection voltage having the same polarities in the earlier half and later half of the selection period is applied to the other of the selected scanning electrodes.
On the other hand, a data voltage, which corresponds to a sum of identity values between the polarity (+1 representing the positive side and -1 representing the negative side) of the selection voltage applied to the scanning electrode during the selection period and the display data (-1 representing power-on and +1 representing power-off) on the scanning electrode, is applied to the data electrode.
The voltage level applied to the scanning electrode is expressed as follows: ##EQU1## where Voff is an effective voltage value applied to a liquid crystal at a display-off period.
On the other hand, the voltage level applied to a data electrode is expressed as follows: ##EQU2## where Voff is an effective voltage value applied to a liquid crystal at a display-off period.
FIG. 4 illustrates an example of a drive voltage's waveform when the liquid crystal panel displays as shown in FIG. 3 using the above-mentioned driving method. The horizontal axis shown in FIG. 3 corresponds to the position of a data electrode Xi (i=1 to n) and the vertical axis shown in FIG. 4 corresponds to a time.
In a passive matrix liquid crystal panel, a voltage applied to a liquid crystal cell corresponds to a potential difference between a scanning electrode and a data electrode. In the use of the plural line selection driving system, the voltage waveforms applied to the points A (display-off), B (display-on) and C (display-on) of the liquid crystal panel of FIG. 3 are shown, for example, in FIG. 5. The effective value of a voltage applied to the liquid crystal panel corresponds to meshed portions in the figure. The effective value of the voltage at the point A (display-off) for the selection period is lower than those of the point B (display-on) and the point C (display-on). These points have the same effective value of voltage for a non-selection period.
Since the light transmittance of the liquid crystal varies depending on the effective value of the applied voltage, on and off of the display of the liquid crystal panel can be controlled by the effective value of voltage in the selection period.
However, in the prior art of the plural line selection driving system when it is actually operated, a distortion of the scanning electrode voltage waveform which is called crosstalk due to, for example, the liquid crystal capacitance or the resistance component of the electrode, occurs when a waveform applied to a data electrode changes its shape. This distortion affects differently depending on the effective value of voltage to some columns during the non-selection period. In an actual case shown in FIG. 6, in the comparison with Points B and C both in the display-on state, the effective value during the non-selection period at the point C increases more than that of an ideal state, but decreases at the point B. For that reason, the point C has a higher transmittance.
For example, in the 2-line selection driving system, when the vertical line as shown in FIG. 3 is displayed, the distortion due to crosstalk occurs once every 1t period. The effective value of voltage is shifted due to the distortion at a high frequency of once every 2t period. For that reason, the display unevenness which occurs at above and below of the vertical line pattern becomes apparent, thus causing image deterioration.
In order to relieve the display unevenness, the orthogonal functions used in the driving system can be combined, for example, as shown in FIG. 7. In this case, as shown in FIG. 8, the crosstalk occurrence frequency is once in 2t period and the shifting of the effective value of voltage occurs once in 4t period.
However, the above-mentioned orthogonal function combination cannot completely remove the display unevenness and the frequency of crosstalk occurrence and effective value of voltage shift occurrence are considered to be too high.
Considering the waveform of a voltage applied to the scanning electrode while neglecting the above mentioned crosstalk for time being, there is a difference in amount of dullness of a waveform when the selection voltage changes as shown in FIG. 9, because the selection voltages of two selected electrodes are changed in a different number of times with reference to each other in the prior art driving method. As a result, the effective value of voltage applied to a liquid crystal is varied on some rows of a display panel so that a display variation occurs horizontally, thus causing image deterioration.
In the plural line driving method, the selection voltage applied to the scanning electrode, as well as the voltage level of the data voltage, are decided by the orthogonal function values. For that reason, the orthogonal function used in the driving method affects on factors regarding image quality besides the display unevenness. Hence, it is important to set the orthogonal function with which the display quality of a liquid crystal panel in total becomes as good as possible.
Next, a prior art configuration that influences power consumption of a liquid crystal display device will be explained.
The voltage averaging method described in "Liquid Crystal Device Handbook", pp. 395-399, has been widely used as a driving system for a liquid crystal display device having a passive matrix liquid crystal panel. In this method, the selection scanning voltage is sequentially applied to the scanning electrodes one by one each of which corresponds to a row of the liquid crystal panel at every scanning period, and then the same operation is again repeated after all the scanning electrodes have been scanned for one frame period. A data voltage corresponding to a display data value, and with respect to a non-selection scanning voltage acting as the center is applied to data electrodes each of which corresponds to a column in the liquid crystal display. Moreover, an alternating operation is performed to reverse the polarity of the liquid crystal applied voltage every fixed period.
On the other hand, the plural line selection driving method disclosed in Japanese Patent Laid-open Publication (Tokkai-Hei) No. 6-67628 is listed as a driving system for a liquid crystal display device having a passive matrix liquid crystal panel. In this method, the selection scanning voltage corresponding to the orthogonal function (e.g. Walsh function) is sequentially applied to scanning electrodes corresponding to a column of the liquid crystal panel so as to be scanned every plural scanning electrodes. When all scanning electrodes have been scanned for a duration called one frame period, the same operation is again repeated. The data voltage, which corresponds to the identity value between the orthogonal function value of a selectively scanned line and a display data value, is applied to the data electrode corresponding to a column of the liquid crystal display.
In this case, according to the voltage averaging method, since the voltage of the data driver and the voltage of the scanning driver are shifted near to the selection voltage of the scanning driver, the output amplitudes VLCD of the data driver is equalized. The equalized value is provided by a following formula (5): ##EQU3## where N is a number of the scanning electrodes.
In contrast, in the plural line selection driving system, the output amplitude Vg of the data driver as well as the output amplitude Vf of the scanning driver are expressed the following formulas (6) and (7): ##EQU4## where m is a number of lines to be selected at one time, and N is a number of the scanning electrodes, and Voff is the effective value of voltage applied to the liquid crystal at a display-off time, usually between 2.0 to 2.5 volts.
In the plural line selection driving system, when the line number m is relatively small, for example, the number m of selection lines is 1, the number N of scanning electrodes is 240, and the effective value of the applied voltage Voff at a display-off time is about 2.2 volts, the formula (6) provides that the amplitude of the data driver is about 3.3 volts. Hence, compared with the amplitude of the data driver in the voltage averaging method (25 volts arrived at using the formula (5)), the amplitude of the data driver can be significantly decreased. Since the power of the data driver is larger than that of the scanning driver, the plural line selection driving system is a low power consumption driving system, compared with the voltage averaging system.
In the plural line selection driving system, although the output amplitude of the data voltage driving means (hereinafter, referred to as a data driver) is small with a small value of the selection line number m, the output amplitude of the scanning voltage driving means (hereinafter, referred to as a scanning driver) becomes large. The output amplitude of the scanning driver increases as the scanning electrode number N increases. Hence, there is a problem that the scanning driver becomes difficult to manufacture because of the withstand voltage problem which depends on the number N of the scanning electrodes.
On the other hand, when the number m of selection scanning lines which are selected simultaneously is large, the difference in amplitude levels between the selection scanning voltage and the display data voltage decreases. For that reason, even if the number N of scanning electrodes is increased, the withstand-voltage of the scanning driver can be lowered sufficiently for manufacturing. However, since the amplitude of the display data voltage increases, the withstand-voltage of the data driver increases. For example, when the withstand-voltage exceeds 5 volts, integrated circuits manufactured embodying the low withstand-voltage standard logic process cannot be used. Hence, the manufacturing cost increases sharply, and a problem arises.
Measuring the liquid crystal module which employs the voltage averaging method in a practical condition, the data driver and the scanning driver are the same drive voltage, but a ratio of the drive current in these drivers is about 10:1. For that reason, in order to reduce the power consumption of the whole liquid crystal module, it is an important problem to reduce the drive voltage of the data driver further, namely to lower the amplitude of the display data voltage.
Moreover, there is the problem that the increased number m of scanning lines which are simultaneously selected as well as the increased scale of the arithmetic-logic circuitry that calculates the identity values between the orthogonal function and the display data result in increasing power consumption and a manufacturing cost of the circuit.